Process for coating aluminum

ABSTRACT

A PROCESS FOR FORMING AN ADHERENT, ALKALINE-RESISTANT COATING ON AN ALUMINUM SURFACE COMPRISING TREATING THE SURFACE WITH AN AQUEOUS ALKALINE SOLUTION OF A SOLUBLE ALKALINE EARTH COMPOUND SUCH AS CALCIUM CHLORIDE. THE COATING IS PRIMARILY COMPLEX DOUBLE SALTS OF CALCIUM AND ALUMINUM.

United States Patent O 3,726,721 PROCESS FOR COATING ALUMINUM Henry J. Wittrock and Bernard R. Baker, Pleasanton, and Albert H. Bushey, Danville, Calif., assignors to Kaiser Aluminum & Chemical Corporation, Oakland, Calif. No Drawing. Filed July 21, 1971, Ser. No. 164,816 Int. Cl. C23f 7/06 U.S. Cl. 1486.27 10 Claims ABSTRACT OF THE DISCLOSURE A process for forming an adherent, alkaline-resistant coating on an aluminum surface comprising treating the surface with an aqueous alkaline solution of a soluble alkaline earth compound such as calcium chloride. The coating is primarily complex double salts of calcium and aluminum.

BACKGROUND OF THE INVENTION Over the years many processes have been developed to coat metallic aluminum and its alloys with a corrosion and/ or abrasion-resistant material. A frequently employed process is a process of anodizing wherein the aluminum article is immersed in an aqeuous electrolyte and subjected to electrolysis forming an anodic oxide coating. Generally, the electrolytes contain various types of acids such as sulfuric acid, oxalic acid, phosphoric acid, chromic acid, sulfosalicyclic acid, and the like. The anodized coatings have been found widespread use and provide a high degree of corrosion and abrasion resistance. Chemical conversion coatings have also been employed to form an aluminum oxide coating on aluminum surface. The process normally involves subjecting the aluminum surface to an oxidizing solution such as sodium chromate. Most of the chemical conversion coating processes are pretreatments for the subsequent painting of the aluminum article, although some of the thus treated articles are used in architectural applications. Most of these processes involve treating the aluminum surface in an acidic solution.

McCulloch, in US. 1,751,213, describes a process wherein an adherent coating is formed on aluminum by subjecting the aluminum surface to a slightly alkaline solution containing lime and calcium sulphate. The film produced is described as having superior dielectric properties. On a small scale, the process described by McCulloch does produce a dense, adherent coating on aluminum. However, when this process is scaled up to large commercial-size units, it was found that only a powdery, nonadherent film was formed on the aluminum surface.

DESCRIPTION OF THE INVENTION The present invention provides an improved process for coating an aluminum surface which, on a large scale, forms a tenacious, alkaline-resistant, non-glaring coating. The process comprises subjecting an aluminum surface to an alkaline solution of alkaline earth metal salts selected from a group consisting of soluble calcium, magnesium, barium and strontium salts. Suitable salts include the chlorides, nitrates and acetates. The concentration of the alkaline earth salt must be greater than 0.1 M, preferably greater than 0.2 M. An excessive amount of alkaline earth metal salt does not detrimentally affect the process. The alkalinity of the solution is generated from sodium hydroxide, potassium hydroxide or lithium hydroxide, and

3,726,721 Patented Apr. 10, 1973 these compounds must be at a concentration level exceeding 0.005 M. An excessive amount of alkali metal hydroxide causes the precipitation of the alkaline earth hydroxide and care should be taken so that the alkaline earth ion concentration does not fall below 0.11 M. The process is most easily controlled by maintaining the alkaline earth metal concentration and the 'hydroxyl ion concentration at the point where the alkaline earth hydroxide begins to precipitate, although the process does operate at levels considerably below the saturation point for the alkaline earth metal hydroxide. The temperature of the bath must be maintained above F., preferably from F. to the boiling point. The process is applicable to pure aluminum, commercial pure aluminum and aluminum alloys containing more than 50% aluminum. The term aluminum as used herein refers to both the pure rnetal, commercial pure aluminum and the alloys thereof containing more than 50% aluminum.

In accordance with this invention, the aluminum surface is subjected to the action of a highly alkaline solution of alkaline earth metal salts such as calcium chloride at elevated temperatures. Initially, hydrogen gas evolves from the bath for a short period of time which indicates that the aluminum surface is being etched. However, after a short period, the gas evolution ceases and the coating begins to form. The process is self-arresting in that hydrogen evolution is slowed as the coating is formed. The speed of the coating formation is increased with increased temperatures, increased pH, and increased alkaline earth metal salt concentrations. The coatings formed have a normal thickness of about 0.1 mil. Normally, the thickness of the coating is not affected by the length of time the aluminum surface is treated, i.e. after a nominal 0.1 mil thick coating is formed, all reactions usually cease. Process times range from ten seconds to 60 minutes, usually from 2 to 15 minutes. Extended treatments such as up to 4 hours have no detrimental effect on the coating. After treatment, the surface is rinsed with water to remove any deliquescent materials such as calcium chloride. Baking the coating at elevated temperatures to dry the coating is unnecessary after rinsing.

The coatings of the present invention are characterized by high resistance to acid environments and, for all practical purposes, are completely inert to alkaline attack. In a Waterfog test (ASTM designation D1735-62), the coatings of the present invention have been immersed for nine months with no evidence of attack. An untreated, bare aluminum control piece experienced severe staining and rnottling during the same period. In a SWAAT test (twohour cyclic acidified sea water, US. Navy specification QQA/OOZSO/ 19/20), the coatings of the present invention survived 144 hours (72 cycles) exposure with no visible attack, and after 216 hours exhibited 2 or 3 tiny blisters. Untreated, bare aluminum normally experiences severe etch pitting during the same period of exposure. The coat ings of the present invention are sufficiently abrasionresistant to withstand normal handling and transportation.

When processing aluminum surfaces on a large scale in accordance with the present invention, the alkaline earth metal ion concentration in the aqueous solution must exceed 0.11 M to consistently produce a uniform, tenacious, alkaline-resistant coating. Below 0.11 M, particularly below 0.045 M, a powdery coating is formed which has neither the tenacity nor the corrosion-resistance of the coatings formed in accordance with the present invention. It is believed that at the lower calcium ion and hydroxyl ion concentrations, a transient depletion of these ions occurs at the aluminum surface resulting in the reaction of aluminum species with the calcium ions occurring at some distance from the aluminum surface and the formation of a powdery coating on the surface. Because of the high hydroxyl ion and calcium ion concentration of the present invention, the depletion of these ions during processing is not a problem and tenacious coatings can be consistently produced. The prior art processes, such as McCulloch, depended upon the dissolution of the calcium salt to replenish the calcium ions and hydroxyl ions which are consumed in the reaction. The dissolution of the compound and the mass transport of ions to the aluminum surface is apparently too slow in these latter processes in commercial sized operations to effect a tenacious alkaline-resistant coating.

With the process of the present invention, the pretreatments characterized by the prior art processes, such as anodizing and chemical treatments, are unnecessary. The aluminum articles can be treated as received in accordance with the present process, and in all cases, an adherent alkaline-resistant coating is obtained. However, for a more uniform surface appearance, it may be desirable to degrease the aluminum surface with an inhibited alkaline cleaner and the like.

The coating composition from X-ray spectroscopy is essentially an array of compositions from the simple alkaline earth aluminates to the complex double salts. The following compounds have been identified in coatings formed in a calcium chloride-sodium hydroxide bath:

3CaO A1 61-1 9; 3CaO A1 0 -CaCO llH O 3CaO -AI O -CaCl lZH O; 3CaO -Al O -CaCl 13H O Although the waters of hydration for the double salts are described above as 11, 12 and 13, respectively, the amount of water of hydration can vary from 8 to 15. The adherent coatings of the present invention are primarily complex double salts and are quite similar in composition to the powdery coatings. However, the crystal size of the adherent coating is much smaller than the powdery coating. The carbonate double salt is formed due to the adsorption into the solution of carbon dioxide from the atmosphere.

The process of the present invention is most conveniently carried out by submerging the aluminum surface to be treated into a bath described above. The coating can also be formed by spraying the alkaline solution of the present invention onto the aluminum surface. However, in this instance, the concentration of the alkaline earth will normally range from about 1.6 to 2.3 M at a temperature of from 170 to 190 F. To minimize depletion of the active ingredients in the bath by carbonate formation, it is preferred to spray the solution with an airless jet.

During the operation of the present process, the concentration of salts such as sodium chloride builds up in the bath, but this increased concentration has no noticeable deleterious elfects upon the process and/or the products made therefrom. Aluminum is continuously being dissolved by the process of the present invention, but no significant amount of aluminum is found dissolved in the bath. However, one or more forms of alumina and alkaline earth aluminate are found in the undissolved sludge on the bottom of the tank.

Due to the consumption of hydroxyl ions and alkaline earth ions in the reaction which forms the coatings, additions such as sodium hydroxide and calcium chloride are normally made at periodic intervals to maintain the desired concentration of these ions in solution. Due to the evolution of hydrogen during the initial portion of the process, it is desirable to tilt the aluminum Workpiece to prevent the formation of gas pockets on the under surface of the aluminum workpiece.

The following example is given to illustrate an embodiment of the present invention and is given for purposes of illustration not limitation.

A 12,000-ga1lon tank was charged with 3l00-pounds of calcium chloride flakes analyzing calcium chloride, and 14-gallons of 50% sodium hydroxide solution (6.33- pounds sodium hydroxide/ gallon) and filled to near capacity with tap water to provide a solution containing about 25 grams/ liter calcium chloride and having a pH of about 12.3. One hundred tons of extrusions formed from 6061 aluminum alloy (Aluminum Association designation) were treated over a two-week period in the manner in accordance with the present invention by submerging the extrusions in the bath at a temperature of from 209 to 214 F. for a period of from 5-10 minutes. After treatment, the extrusions were rinsed with cold water to remove excess calcium chloride and prevent deliquescence. In all cases, an adherent alkaline-resistant coating was formed on the aluminum extrusions.

In another series of tests, aqueous solutions were prepared containing, respectively, 21.4 grams/liter MgCl and 1 gram/liter NaOH, 35.7 grams/liter SrCl and 1.0 gram/liter NaOH, and 47 grams/liter BaCl and 1.0 gram/liter NaOH. Each of the solutions were heated to boiling and 6061 aluminum alloy extrusions (Aluminum Association designation) were submerged in each of the solutions for a period of about 10 minutes. In each case, a tenacious coating was formed.

The color of the low gloss coatings formed by the present invention ranges from a pearl grey to a dark grey depending upon the alloy composition of the base metal. The base color of the coating can be modified by incorporating coloring agents into the bath itself. For example, if a white coating is desired, finely ground titanium dioxide may be incorporated into the bath itself. The gloss of the coatings of the present invention at 45 generally ranges from about 3 to 12 as measured with a Model 670 Photovolt Gloss Meter. A black carrara glass is used as a standard of 100. This gloss is to be compared with the gloss of bare aluminum which generally runs well above 50, e.g. 400 or 500.

If a bare steel container is used to contain the bath of the present invention, care should be taken that no contact be made between the aluminum surface to be treated and the steel walls of the container. If the aluminum workpiece is in contact with the steel walls of the container, only a powdery coating is obtained, apparently due to the high potential difference between the two metals. It has also been found that when several different aluminum alloy types, electrically coupled, are processed at the same time, the coating preferentially forms on the most electronegative alloy, particularly when there is a large potential difference between the alloys.

It is obvious that various modifications and changes can be made to the process of the present invention without departing from the spirit of the present invention or the scope of the appended claims.

What is claimed is:

1. A process for forming an adherent coating on an aluminum surface comprising contacting the surface with an aqueous solution at a temperature greater than F. consisting essentially of more than 0.005 gram-mole/ liter of an alkali metal hydroxide selected from a group consisting of sodium hydroxide, lithium hydroxide and potassium hydroxide, more than about 0.11 gram-mole/ liter of a soluble alkaline earth metal salt, and the balance water the amount of said alkali metal hydroxide being controlled so as to maintain at least 0.11 gram-mole/liter of said alkaline earth metal salt in solution.

2. The process of claim .1 wherein the alkaline earth metal salt is selected from the group consisting of calcium chloride, calcium nitrate and calcium acetate.

3. The process of claim 2. wherein the calcium salt concentration ranges from 0.1!1 gram-mole/liter to 6.2 gram-moles/liter.

4. The process of claim 1 wherein the temperature of the solution is maintained from about 150 F. to the boiling point of the solution.

5. The process of claim 1 wherein the solution is sprayed onto said aluminum surface with an airless jet.

6. The process of claim 1 wherein the solution is in contact with the aluminum surface from about ten seconds to 60 minutes.

7. The process of claim 1 wherein coloring agents are added to said solution to incorporate said agents into the coating formed.

8. The process of claim 1 wherein the alkaline earth metal salt is a magnesium salt.

9. The process of claim 1 wherein the alkaline earth metal salt is a barium salt.

10. The process of claim 1 wherein the alkaline earth metal salt is a strontium salt.

References Cited UNITED STATES PATENTS 

